Construction lasers with a rotatable laser beam are used primarily in the constructional industry as an auxiliary means for tracing horizontal and vertical lines on floors, ceilings and walls or to mark points or objects, to fix paths or reference lines, or to determine horizontal or vertical planes, or planes at defined inclinations on walls, ceilings and floors, which helps with the orientation or positioning along objects. Rotating construction lasers can be used, e.g. for aligning walls, windows or door frames and for the determination of the course of plumbing or electrical installations.
A conventional prior art rotational constructional laser comprises a laser unit located in a housing, an emitted laser beam of which being deflected by about 90° by a deflection device. The housing usually comprises optically transparent windows or cut-outs, which enable the laser to be emitted to the outside. The deflection device rotates about an axis that in general is coincident with the optical axis. Upon rotation of the deflection device, the laser beam, which, as a result of its deflection, propagates transverse to the rotational axis of the deflection device, describes a plane in such a way that a visible or detectable path or line is generated on a surface.
To enable an alignment of the path according to predefined or given settings the laser unit generally is mounted tiltably in two mutually perpendicular directions in relation to a housing that encloses the laser unit. It is well known from prior art that rotating construction lasers can be levelled with the help of a reference vial, in particular two orthogonally aligned reference vials, comprising sensor means for the determination of an absolute grade of the optical core module.
Conventionally, for controlling the grade of a rotating construction laser and its horizontal alignment a grade mechanism is used, which comprises a tilt sensor, e.g. a reference vial, that is mounted on a tiltable grade arm, respectively two orthogonally aligned tilt sensors on one grade arm or platform. Alternatively, the European patent application EP 09177262.4 describes the use of two tilt sensors on two orthogonally aligned grade arms.
In prior art there are various documents available that generally disclose a rotational laser, the inclination of which can be adjusted by a grade mechanism of this design—for instance the document U.S. Pat. No. 5,485,266. It is further known from this document that the tilting of the grade arm is run by a motor, in particular a stepper motor, which rotatably drives a lead screw that is connected with the grade arm by means of a nut. The grade mechanism employs an encoder, either in its lead screw mechanism to detect a rotation speed of the lead screw mechanism, or in its driving motor, which rotatably drives the lead screw. In case of a stepper motor the number of steps performed by the stepper motor corresponds to a certain change in the tilting angle of the grade arm. In order to be able to return the grade arm to the home position the number of steps performed by the stepper motor into a certain direction is counted and in particular memorized.
As a result of an accumulated error due to repetitive use of the lead screw mechanism, or a wear of a part of the mechanism due to repetitive use, or a deterioration of a part of the mechanism, errors can occur and cause general deviations in angle setting, so that tilting accuracy cannot be guaranteed any longer. Also, the stepper motor can lose count of the steps performed, e.g. if the device falls to the ground or through other circumstances receives a shock. As a consequence the grade mechanism would not be capable of returning the grade arm to the correct home position any longer. In following applications of the construction laser this would then lead to an incorrect inclination and would make a recalibration necessary.
With a feedback sensor that measures the tilt of the grade arm in at least one position, preferably the home position, it would always be possible to reference back to the original position. None of the solutions for rotational lasers of the above-mentioned prior art documents has a form of feedback-control that allows to confirm a set grade or to show an error or deviation. A feedback sensor of this type is already known from the European patent application EP 1 901 034 A2. This document discloses a feedback sensor system with a light emitting diode (LED) and a pattern plate that are mounted on the nut of the lead screw, together casting a light pattern onto an image reception surface of a charge-coupled device (CCD) array scanner mounted on the lens barrel. This solution, though, has some disadvantages: first of all there is still the possibility that the nut and particularly the connection of the nut with the grade arm might become imprecise. This, unfortunately, reduces the reliability of the feedback sensor. Another disadvantage arises from its complex, and therefore relatively pricey, setup with a pattern plate and a CCD.